1. Field of the Invention
The present invention generally relates to passive infrared (PIR) motion detectors and, more particularly, to low-power PIR motion detectors having no more than one amplification stage and no window comparator.
2. History of the Prior Art
Infrared (IR) radiation is electromagnetic radiation having a wavelengths that are longer than those of visible light and shorter than those assigned to microwave radiation. IR radiation is assigned wavelengths between 0.7 and 300 μm, which equates to a frequency range of approximately 1 to 430 teraherz. Bright sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level. Of this energy, 527 watts is infrared radiation, 445 watts is visible light, and 32 watts is ultraviolet radiation. Although IR radiation cannot be seen by most animals, it can be detected as heat. Snakes of the pit viper family have a unique pit between the eye and nostril on each side of the head that senses IR radiation. All warm-blooded animals, including humans, emit IR radiation. Humans, for example, emit IR radiation having a peak frequency of 9.4 μm. The heat-sensing organs of the pit vipers enable them to locate warm-blooded prey even in total darkness. The pit viper brain sees the IR images from the pits superimposed on the visible image from its eyes.
Pyroelectricity (from the Greek pyr, fire, and electricity) is the ability of certain materials to generate a temporary voltage when they are heated or cooled. Although artificial pyroelectric materials have been manufactured in recent years, the pyroelectric effect was first discovered by Theophrastus, who noted around 314 BC that tourmaline (essentially sodium aluminum borosilicate) attracted bits of straw and ash when heated. The pyroelectric effect is also present in both bone and tendon, as well as in certain tissues within the IR-sensing pits of the viper.
Pyroelectric infrared radiation sensors—both man made, as well as those of the pit vipers—are made of non-centrosymmetric (i.e., not having a center of symmetry), piezoelectric, polar (i.e., having a dipole in each crystal unit) crystalline materials. These materials generate a transitory voltage when heated or cooled. Under static conditions, polar crystalline materials do not display a net dipole moment, as the material's intrinsic dipole moment is neutralized by “free” electric charge that builds up on the surface by internal conduction or from the ambient atmosphere. Polar crystals only reveal their pyroelectric nature when subjected to a change in temperature that momentarily upsets the balance with the compensating surface charge. The change in temperature slightly modifies the positions of the atoms within the crystal structure, such that the polarization of some of the crystals reverses, resulting in a net dipole moment. This reversal of polarization gives rise to a voltage across the crystal. If the temperature stays constant at the new value, the pyroelectric voltage gradually disappears due to leakage current (the leakage can be caused by a variety of factors, including electrons moving through the crystal, ions moving through the air, or current leaking through a voltmeter attached across the crystal).
Passive infrared (PIR) motion detectors have been in use for decades for security and energy saving applications. Manufacturers of the pyroelectric sensor components publish recommended circuitry for amplifying and conditioning the minute electrical signal to a usable level. These reference circuits require substantial signal gain (thousands of times amplification) and typically require multiple stages of amplification. After amplification, the circuits use a window comparator (a pair of comparators to monitor if the signal has exceeded a certain range) to convert the analog signal into a digital signal which indicates occupancy. For some prior art devices, the signal is processed by a microprocessor in order to determine whether the signal indicates the presence of humans or pets, or between benign occupancy and a security breach caused by an intrusion. Thus, signals from PIR sensors are processed only after they have been amplified in order to determine whether certain conclusions can be drawn from the characteristics which the signal possesses. Signals from PIR sensors have, heretofore, not been processed without first subjecting them to multiple high-gain amplification stages and the use of at least one window comparator.
U.S. Pat. No. 5,764,146 to John R. Baldwin, et al. discloses a multifunction passive infrared occupancy sensor that functions as an occupancy sensor for both security intrusion alert and for energy management control systems. U.S. Pat. No. 5,640,143 to Douglas D. Myron, et al. discloses an occupancy sensor that provides improved performance by the inclusion of a microprocessor which controls the sensing transducers and processes the received signal to optimize desired detection performance. The sensor includes a quadrature detection technique and automatic sensitivity adjustment that reduces false detection caused by air flow, hallway traffic and other noise sources. U.S. Pat. No. 7,123,139 to Kevin Sweeney discloses an occupancy sensor for determining whether a room is occupied. The sensor integrates a battery-powered PIR motion detector and a battery-powered Hall Effect switch, each of which communicates wirelessly with a controller. U.S. Pat. No. 7,471,334 to Thomas A. Stenger discloses an outdoor, battery-powered digital camera that includes a passive infrared motion detector that allows the camera to be left unattended, as the detector automatically triggers the camera to take a picture upon sensing the presence of a moving animal. To prolong battery life, the camera goes into a power-saving sleep mode between pictures. The camera's exposure settings are periodically checked, adjusted and stored so that it can take a picture with a fairly recent exposure setting when suddenly awakened by the motion detector.
FIG. 1 is a diagram of typical prior-art general purpose motion detector circuit 100. It uses a low-cost LM324 quad operational amplifier as both a two stage amplifier (IC1A and IC1B) and a window comparator (IC1C and IC1D). Suggested component values are as follows: R1=10KΩ; R2=100KΩ; R3=10KΩ; R4=1 MΩ; R5=1 MΩ; R6=1 MΩ; R7=1 MΩ; R8=1 MΩ; R9=1 MΩ; R10=1 MΩ; R11=10KΩ; R12=10KΩ; C1=10 μf; C2=10 μf; C3=0.1 μf; C4=10 μf; C5=0.1 μf; and C6=1 μf. PIR sensor 101 is connected directly to ground through terminal 2. It is also connected to Vcc at terminal 1. C1 and R1 act as filters between the PIR sensor 101 and Vcc, as even tiny fluctuations in Vcc could perturb the PIR sensor, thereby causing output fluctuations that might well result in false occupancy detection. R2 continually pulls node 2 toward ground so that drops in the sensor output can be sensed by IC1A. Operational amplifiers (op-amps) 1C1A and 1C1B have a gain of 100 each, for a total gain of 10,000. As long as PIR sensor 101 detects no change in IR radiation intensity, operational amplifier IC1A is in a steady state condition, with the voltages at nodes 3, 4 and 5 being roughly equal to the output of the sensor 101, which is typically about 1 volt. However, when the voltage on node 3 changes in response to a change in detected IR radiation intensity by sensor 101, node 5 will reflect a 100-fold signal gain as IC1C attempts to equalize the voltage at nodes 3 and 4. The gain at node 5 is set by the ratio of R3 to R4, which together form a voltage divider. Resistor R5 and capacitor C4 together act to block the DC component of the output signal at node 5. The function of op-amp IC1B is analogous to that of op-amp IC1A. Because the resistance values of resistors R6 and R7 are the same and diodes D1 and D2 have identical threshold voltages, the voltage at node 8 (the non-inverting input of op-amp IC1D) is at Vcc/2. Diodes D1 and D2 are selected so that node 7 is held at 200 millivolts above Vcc and node 9 is held at 200 millivolts below Vcc. Op-amps IC1C and IC1D form a window comparator that responds to signals above 200 millivolts above and 200 millivolts below Vcc/2. This 400 millivolt-wide window is set by the low-current threshold-voltage drops across D1 and D2. In a steady state condition, the output of op-amp IC1B (node 10) is at Vcc/2. However, when the inverting input to op-amp IC1B (node 6) varies sufficiently so that node 10 is outside the 400 millivolt-wide window, the op-amps IC1C and IC1D of the window comparator trip and provide outputs at either node 11 or node 12, either of which is transferred to node 13 through diodes D3 or D4, respectively. Diode D3 isolates node 11 from node 13, and diode D4 isolates node 12 from node 13, thereby preventing unwanted cross-interference between IC1C and IC1D. In any case, diodes D3 and D4 pass only positive transitions into pin 4 of CD4538 CMOS single shot IC2. A timed output on pin 6 of IC2 feeds into NPN transistor Q1, which drives relay RY1. Resistor R10 and capacitor C6 set the time constant that determines how long the relay remains energized after motion is detected. Diode D5 protects IC2 from unsafe voltages generated by the collapse of the magnetic field of the solenoid of relay RY1 when transistor Q1 shuts off the current thereto. All components can operate on 5 to 12 volts. This type of circuit is often used to turn a light on outside a house when motion is detected.
Each stage of signal amplification consumes electrical power. In a line-powered or even many battery-powered devices, the amount of energy required for signal amplification is low enough so as to be negligible. However, in an energy-harvesting system operated by solar power, by a cell or battery charged intermittently by solar power, or simply by a cell or battery, the energy consumed by signal amplification circuitry overwhelms all other energy expenditures in the circuitry. If power-hungry amplification and comparator stages could be eliminated, battery life and operational time during periods of darkness could be extended significantly, with the added benefit of concomitant reduction in system cost and complexity.
The technology disclosed in this application has been incorporated into wireless control products produced by Ad Hoc Electronics LLC under the ILLUMRA trademark. Ad Hoc Electronics, a member of the EnOcean Alliance, has become the largest supplier in North America, of self-powered, battery-free, wireless lighting control and energy management systems. EnOcean GmbH of Oberhaching, Germany is a pioneer in the design and manufacture of energy-harvesting switching and sensor modules. EnOcean's primary technological contribution was the creation of wireless switches and radio modules which operate with minuscule amounts of energy. As a result of this breakthrough, energy-harvesting wireless sensors, of the type produced by EnOcean and its partners, can work where those based on other technologies fail. Energy-harvesting wireless switches and sensors are prime examples of such devices. All ILLUMRA™ products operate using the EnOcean protocol, which is the de-facto standard for energy-harvesting wireless controls. The technology allows energy harvesting ILLUMRA™ transmitters to operate indefinitely without the use of batteries. The motion of a switch actuation, light on a solar cell, or other ambient energy in the environment provide power to ILLUMRA™ transmitters, providing zero-maintenance wireless devices. The ILLUMRA™ product line includes multiple products which operate in the uncrowded 315 MHz band offering greater transmission range than other wireless technologies and minimal competitive traffic.
Given the energy-harvesting, wireless focus of products designed by Ad Hoc Electronics LLC, the minimization of power consumption in those products is essential. In spite of the fact that EnOcean PIR sensors are likely current state-of-the-art low power devices, they, like most other existing designs by other manufacturers, employ 2 amplifiers and 2 comparators. Estimated continuous power consumption of EnOcean's PIR sensors is estimated to be 5 to 10 μamps.